You can't. You stop the attack vectors you can and inform people about the attack vectors you can't do anything about. In this case, making sure the file isn't

You can't. You stop the attack vectors you can and inform people about the attack vectors you can't do anything about. In this case, making sure the file isn't on the device longer than it should be. Keep in mind, when it comes to extracting images, you can't stop a sufficiently informed or sufficiently determined end user, because all of your code runs on a device that's wholly in control by the end user. All you can do is basic due diligence. There is the side problem where the attacker isn't the end user (e.g., malware attempting to extract private images from a phone). At the end of the day though, there is squat all you can do against code that has root.